Inputs

The INCREMENT increment clause is optional.
A positive value will make an ascending sequence, a
negative one a descending sequence. The default value is
one (1).

minvalue

The optional clause MINVALUE
minvalue determines
the minimum value a sequence can generate. The defaults are
1 and -2147483647 for ascending and descending sequences,
respectively.

maxvalue

Use the optional clause MAXVALUE
maxvalue to
determine the maximum value for the sequence. The defaults
are 2147483647 and -1 for ascending and descending
sequences, respectively.

start

The optional START start clause enables the
sequence to begin anywhere. The default starting value is
minvalue for ascending
sequences and maxvalue for
descending ones.

cache

The CACHE cache option enables sequence
numbers to be preallocated and stored in memory for faster
access. The minimum value is 1 (only one value can be
generated at a time, i.e. no cache) and this is also the
default.

CYCLE

The optional CYCLE keyword may be used to enable the
sequence to continue when the maxvalue or minvalue has been reached by an
ascending or descending sequence respectively. If the limit
is reached, the next number generated will be whatever the
minvalue or maxvalue is, as appropriate.

Outputs

CREATE

Message returned if the command is successful.

ERROR: Relation 'seqname' already exists

If the sequence specified already exists.

ERROR: DefineSequence:
MINVALUE (start) can't be
>= MAXVALUE (max)

If the specified starting value is out of range.

ERROR: DefineSequence: START
value (start) can't be <
MINVALUE (min)

If the specified starting value is out of range.

ERROR: DefineSequence:
MINVALUE (min) can't be >=
MAXVALUE (max)

If the minimum and maximum values are inconsistant.

Description

CREATE SEQUENCE will enter a new
sequence number generator into the current data base. This
involves creating and initializing a new single-row table with
the name seqname. The generator
will be "owned" by the user issuing the command.

After a sequence is created, you may use the function
nextval(seqname) to get a new number from the
sequence. The function currval('seqname') may be used to determine the
number returned by the last call to nextval(seqname)
for the specified sequence in the current session. The function
setval('seqname', newvalue) may be used to set the
current value of the specified sequence. The next call to
nextval(seqname) will return the given value
plus the sequence increment.

Use a query like

SELECT * FROM seqname;

to get the parameters of a sequence. As an alternative to
fetching the parameters from the original definition as above, you
can use

SELECT last_value FROM seqname;

to obtain the last value allocated by any backend.

Low-level locking is used to enable multiple simultaneous
calls to a generator.

Caution

Unexpected results may be obtained if a cache setting
greater than one is used for a sequence object that will be
used concurrently by multiple backends. Each backend will
allocate and cache successive sequence values during one
access to the sequence object and increase the sequence
object's last_value accordingly. Then, the next cache-1
uses of nextval within that backend simply return the
preallocated values without touching the shared object. So,
numbers allocated but not used in the current session will
be lost. Furthermore, although multiple backends are
guaranteed to allocate distinct sequence values, the values
may be generated out of sequence when all the backends are
considered. (For example, with a cache setting of 10,
backend A might reserve values 1..10 and return nextval=1,
then backend B might reserve values 11..20 and return
nextval=11 before backend A has generated nextval=2.) Thus,
with a cache setting of one it is safe to assume that
nextval values are generated sequentially; with a cache
setting greater than one you should only assume that the
nextval values are all distinct, not that they are
generated purely sequentially. Also, last_value will
reflect the latest value reserved by any backend, whether
or not it has yet been returned by nextval.

Notes

Use DROP SEQUENCE to remove a
sequence.

Each backend uses its own cache to store allocated numbers.
Numbers that are cached but not used in the current session
will be lost, resulting in "holes" in the sequence.